Fabric faults can be caused by so-called transition operating conditions that result from a change in a weaving pattern. A change in a weaving pattern may for example, require a temporary change in the rated r.p.m. of the main loom drive or a temporary change in the rated operational pressure of the auxiliary nozzles that eject air streams for transporting the weft thread through the weft insertion channel of an air weaving loom.
Conventional weaving methods with features for avoiding of weft defects in the fabric assume, that such fabric defects or faults occur when the loom is started, for example, following a standstill. Such standstill may occur due to an interruption of the weaving operation, for example, for the purpose of removing a weft thread break. Reference is made in this connection to German Patent Publication DE-OS 4,137,681 A1 and European Patent Publication EP 0,567,428 A1, the latter corresponding to U.S. Pat. No. 5,335,698 (Berktold et al.).
German Patent Publication (DE-OS) 4,137,681 A1 (Shinbara et al.), published on May 27, 1992 discloses a method and apparatus for preventing of weft defects in an air nozzle loom. The beat-up edge of the fabric is displaced relative to reed beat-up position in the direction of the warp by an automatic control depending on whether the loom drive performs a slow forward or a slow backward rotation.
European Patent Publication EP 0,567,428 A1 (Berktold et al.), published on Oct. 27, 1993 discloses a method for starting a loom and a loom capable of operating in accordance with the method for the purpose of avoiding start-up defects in the fabric due to an improper weft beat-up during starting of the loom either initially or following any other loom stop, e.g. a stop caused by the need for removing a broken weft thread. During a first start-up phase the beat-up edge of the fabric is kept at a spacing from the beat-up position of the reed. At this time the supply of weft thread is blocked, several so-called empty weaving cycles are performed. No shed is being formed at this time. Thereafter, the normal shed forming control is resumed in accordance with the weaving normal program.
Fabric faults in the form of weft defects, however, occur not only during a restarting of the loom following a standstill, but also during normal weaving when changes are made that affect the operational parameters of the loom, for example changing the type of weft thread to be processed. Such operational parameters involve primarily the r.p.m. of the main loom drive and the operational pressure of the auxiliary weft insertion nozzles. These parameters must be automatically changed to avoid conditions that could adversely affect the quality of the fabric, if during such a phase of change a weft thread insertion takes place. Thus, it is known that the weft thread insertion of weft threads of different qualities, for example fiber yarns or chenille yarns, into the loom shed of an air nozzle loom requires different pressure parameters at the auxiliary weft insertion nozzles. Similarly, changes in the loom drive r.p.m. may be necessary in the sense of an r.p.m. reduction or an r.p.m. increase.
It is also known in this connection that during each change of an operating parameter during a weaving operation, short duration so-called transition operational conditions occur. The term "transition operational conditions" is intended to refer to changes occurring during a time duration when the loom control automatically switches, for example, from a first effective rated operational pressure of the auxiliary nozzles to a second such pressure, to a third such pressure, or to a still further effective rated operational pressure. A respective transition parameter becomes effective during the switching. A required change in the r.p.m. of the main loom drive is also to be viewed under this aspect since it causes a respective transition operational condition.
It has been ascertained on the basis of a series of tests that a weft thread insertion that takes place during the duration of a transition operational condition, leads to fabric faults or defects and/or to a stopping of the loom, because between the weft insertion and the other functional operations of the loom there is no synchronism during these transition operational conditions. Thus, there is room for improvement.